Electronic, structural, and magnetic properties of the half-metallic ferromagnetic quaternary Heusler compounds CoFeMn$Z$ ($Z=\text{Al}$, Ga, Si, Ge)

The quaternary intermetallic Heusler compounds CoFeMn$Z$ ($Z=\text{Al}$, Ga, Si, or Ge) with $1:1:1:1$ stoichiometry were predicted to exhibit half-metallic ferromagnetism by ab initio electronic structure calculations. The compounds were synthesized using an arc-melting technique and the crystal structures were analyzed using x-ray powder diffraction. The electronic properties were investigated using hard x-ray photoelectron spectroscopy. The low-temperature magnetic moments, as determined from magnetization measurements, follow the Slater-Pauling rule, confirming the proposed high spin polarizations. All compounds have high Curie temperatures, allowing for applications at room temperature and above.

Figure 1

Crystal structure of the quaternary $Y$-type Heusler compound CoFeMnSi and related primitive cell. There are three nonequivalent supercells of CoFeMnSi in the $Y$-type structure, depending on the occupation of the four different lattice sites $4a$ $(0,0,0)$, $4b$ $(1/2,1/2,1/2)$, $4c$ $(1/4,1/4,1/4)$, and $4d$ $(3/4,3/4,3/4)$. As deduced from ternary Heusler compounds, Co and Fe should occupy the lattice sites $4c$ and $4d$, and Mn and Si should occupy $4b$ and $4a$. Si atoms are gray, Co atoms are orange (medium gray), Fe atoms are brown (dark gray), and Mn atoms are red (atoms with larger radius). The ratios of atomic radii correspond to the factual values of the atoms.

Figure 2

Powder XRD and Rietveld refinements of CoFeMn$Z$ [$Z=\text{Al}$ (a), Si (b), Ga (c), and Ge (d)]. The measurements were carried out at room temperature using Mo $K\alpha$ radiation.

Figure 3

Band structures and densities of states of CoFeMnAl: (a) minority bands, (b) densities of states, and (c) majority bands. Majority- and minority-spin densities are denoted by $\uparrow$ and $\downarrow$, respectively. The irreducible representations of the bands are given for the $\Gamma$ point.

Figure 4

Band structures and densities of states of CoFeMnSi: (a) minority bands, (b) densities of states, and (c) majority bands. Majority- and minority-spin densities are denoted by $\uparrow$ and $\downarrow$, respectively. The irreducible representations of the bands are given for the $\Gamma$ point.

Figure 5

Band structures and densities of states of CoFeMnGa: (a) minority bands, (b) densities of states, and (c)) majority bands. Majority- and minority-spin densities are denoted by $\uparrow$ and $\downarrow$, respectively.

Figure 6

Band structures and densities of states of CoFeMnGe: (a) minority bands, (b) densities of states, and (c)) majority bands. Majority- and minority-spin densities are denoted by $\uparrow$ and $\downarrow$, respectively.

Figure 7

Magnetic properties of CoFeMn$Z$ ($Z=\text{Al}$, Si, Ga, and Ge). The field-dependent measurements were performed at $T=5$ K. The inset shows temperature-dependent magnetic moments in the high-temperature range of the Curie temperature.

Figure 8

Spectra of the shallow core states of CoFeMn$Z$ ($Z=\text{Si}$, Ge). The excitation energy was $h\nu =7.9380$ keV. The spectra are normalized to the maxima for easier comparison; (b) shows an enlarged view of the Mn $3s$ states.

Figure 9

Co, Fe, and Mn $2p$ core levels of CoFeMn$Z$ ($Z=\text{Si}$, Ge). The most important satellites (see text) are marked by arrows. The photon energy was $h\nu =5.9534$ keV.

Figure 10

HAXPES valence-band spectra of CoFeMnSi and CoFeMnGe. The excitation energy was set at $h\nu =7.9380$ keV. For easier comparison, the spectra are normalized to their maxima.

Figure 11

Valence-band spectra of CoFeMnGe at different excitation energies.